Method for controlling the surface state of one face of a solid and the associated device

ABSTRACT

A method and apparatus for checking the surface state of one face (2) of a solid (1) in order to locate shape defects which may be present therein. The observation of the face to be checked takes place by means of photography using a large field video camera (3) and a small field video camera (4). The size of the located defects is measured by an optoelectronic sensor or probe (11). The apparatus can be controlled by an operator or can have automatic control.

The present invention relates to a method for controlling or checkingthe surface state of one face of a solid. It also relates to a devicefor performing this method and intended both for equipment assisting anoperator and for an automatic control installation.

BACKGROUND OF THE INVENTION

For various reasons, it is often necessary to control, inspect or checkthe surface state of one or more faces of a solid and this is moreparticularly the case with laminated nuclear fuel.

One variety of nuclear fuel for experimental reactors is constituted byaluminium sandwich plates with a core made from a mixture of uranium andaluminium. The production process of said plates consists of laminatingtogether four components, namely a compacted aluminium and uranium coremounted in a frame and covered by two plates forming a cover.

Although highly industrial, this production process requires severalmanual operations and know-how, so that there are variations in thequality of manufacture. However, as these products are particularlysensitive, they require very precise characteristics imposed by theusers. These requirements concerning quality give rise to internal andexternal controls of the fuel plates.

The internal control takes place by special machines using ultrasonicsor X-rays. The control takes place automatically and gives rise toreports combining quantitative and calibrated measurements.

The external control has hitherto been performed visually by qualifiedoperators. The procedure involves observing two surfaces of a plateunder glancing light, whilst manipulating the plate and locating orpinpointing surface defects. The latter are holes or scratches, whosedepth must not exceed 100 μm. When a surface defect appears suspect tothe operator, he places the plate beneath the objective of a microscopeand evaluates the maximum depth of the hole or scratch previouslylocated. Plates having characteristics falling outside the standard aredisposed of as waste. Certain users have particularly strictrequirements, which requires a supplementary control performed byanother team.

This external control, which is entirely the responsibility of humanevaluation, suffers from a number of disadvantages. It is fastidious andtiring due to the concentration and acuity required. Its reliability isdependent on the state of vigilance, which varies in time and betweenindividual operators. In the case of a second control, when the qualitytends towards 100%, it remains virtually impossible to maintain thenecessary vigilance for detecting the very rare defect or objectivelyimpose acceptance limits. In addition, visual measurements offer noquantitative and objective basis for the support of a control orinspection report.

The invention makes it possible to improve the control of surface statesand is applicable both to equipment used by an operator and to anautomatic control installation. The observation conditions are improvedby photographing the surfaces to be controlled, said photographs eitherbeing presented on a video screen at a control station, or are digitallyprocessed in the case of an automatic control installation. The use ofan optoelectronic probe makes it possible to obtain objective andquantitative depth measurements, which can be recorded.

SUMMARY OF THE INVENTION

The invention therefore relates to a method for controlling the surfacestate of one face of a solid for locating shape defects liable to belocated there and comprising:

observing the face of the solid in order to locate areas liable toconstitute defects,

observing said areas using optical magnification means,

evaluating the size of these areas to determine, by comparison with agiven limit size implying the existence of a defect, if said areas areor are not defects,

characterized in that:

the observations take place by photography,

the evaluation of the size of the areas liable to constitute defectstakes place by measurement using an optoelectronic probe.

The observation of the face of the solid advantageously takes placeunder glancing, multidirectional illumination.

Preferably, photography or filming takes place by video and in twosuccessive stages, a first or so-called large field analysis stage makesit possible to rapidly pinpoint all the areas liable to constitutedefects and a second or so-called small field analysis stage, which onlyapplies to the areas pinpointed in the first stage, the small fieldanalysis constituting the observation by the magnification means.

The measurement performed by means of the optoelectronic probe can berecorded.

The invention also relates to a device for performing this controlmethod comprising:

means for the reception of the solid permitting a presentation of theface of the solid to be observed,

means for illuminating said face,

a large field video camera for observing said face,

a small field video camera for observing said face,

means for processing the output signals supplied by the video cameras,said processing means supplying informations on the shape defects liableto occur on said face,

an optoelectronic probe controlled by control means receiving saidinformations.

This device can also incorporate means for eliminating the dust liableto be present on said face.

The reception means can comprise a plate for the translation of thesolid permitting the displacement of said face in accordance with twocrossed axes.

The translation plate can be used for the displacement of the said facein accordance with one of the two axes for the large field video cameraand according to both axes for the small field video camera and for theoptoelectronic probe.

With a view to a control by an operator, the means for processing theoutput signals supplied by the video cameras can comprise two monitors,one for displaying the view filmed by the large field video camera andthe other for displaying the view filmed by the small field videocamera.

In this case, the device can incorporate means for displaying valuesmeasured by the optoelectronic probe.

The reception means incorporating a plate for translating the solidpermitting the displacement of said face in accordance with two crossedaxes, the device can incorporate means for controlling said plate inaccordance with these axes operating in the manual mode or in theautomatic mode.

It can incorporate means for recording positions of areas liable toconstitute shape defects and measurements of the probe.

With a view to an automatic control, the device can be equipped with adata processing control system, which processes the output signalssupplied by the video cameras, locates on the basis of these signals theareas liable to constitute shape defects, controls the optoelectronicprobe and analyses the measurements given by the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein show:

FIG. 1 the method for controlling one face of a plate for pinpointingany shape defects according to the invention.

FIGS. 2 and 3 respectively profile and plan views of a plate to becontrolled illuminated with glancing light in one direction.

FIG. 4 a block diagram of a device for performing the control methodaccording to the invention and usable for assisting an operator.

FIG. 5 a block diagram of a device for performing the control methodaccording to the invention and usable for automatic control purposes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plate and it is wished to control or check the surfacestate of its face 2 by firstly observing the said face.

According to the invention this observation takes place by means ofphotography or filming. One satisfactory solution consists of using twovideo cameras, one for performing a large field analysis and the otherfor performing a small field analysis.

The large field camera 3 makes it possible to pinpoint and locate eachpotential defect rapidly and with an accuracy of e.g. approximately 200μm. The small field camera 4 only processes the zones of the large fieldimage having suspected areas. It permits a very accurate analysis andlocation, e.g. with a precision of approximately 20 μm.

The face to be observed is illuminated by a glancing, multidirectionalillumination equipment constituted by several light sources creating ahomogeneous illumination. The latter acts on the discontinuities of thesurface state by overillumination or shadow, forming local contrastswhich would constitute location references. FIGS. 2 and 3 illustratethis type of illumination for a single lamp 5. In this case, the camera3 intersects an image of the defect 6 exposed to said illumination andcomprising a dark area 7 and a light area 8. In practice, use will bemade of four light sources arranged in perpendicular directions.

In this embodiment, the large field video camera 3 covers the width ofthe plate 1, i.e. a surface such as that carrying the reference numeral9 in FIG. 1 and which can be 100 mm×70 mm. The small field video camera4 covers a smaller surface 10 than the surface 9 and which can be 12mm×9 mm.

The device incorporates an optical probe or sensor 11 of the focodynetype (i.e. control of the focal spot of a laser diode), whosemeasurement beam diameter is much smaller than the size of the areasconsidered as defects.

The type of illumination used gives excellent results on cavity defects,but is very sensitive to dust which can be deposited on the plate. Thus,use is made of a dust elimination unit, e.g. by brushing and suction.

The plate is placed on a translation plate member 12 having twoperpendicular, motorized axes X and Y making it possible to accuratelyknow the position of the plate with respect to a reference point. It ispossible for this purpose to use the rotation angle of the drive motorsfor the axes or shafts or an incremental coder. The displacement of theplate member 12 is regulated by an axis or shaft control member.

The plate can be maintained on the plate member 12 by vacuum. It can bedisplaced according to the axis X under the large field camera 3 andaccording to the axes X and Y under the small field camera 4, as well asunder the probe 11.

For an operator-based control station corresponding to the diagram ofFIG. 4, the device can be completed by two different video monitors, onemonitor 13 for the image transmitted by the large field camera 3 and onemonitor 14 for the image transmitted by the small field camera 4. Adisplay device 15 permits the reading of the values measured by theprobe 11.

The control then takes place in the following way. The operator observeson the "large field" video monitor the illuminated plate and made tomove rapidly by the shaft control member 18 in front of the fixed, largefield camera 3. When it detects a suspect area, it stops the movementand points out the located point on the screen 13 and then continues therapid run up to the end of the plate.

On return, the plate automatically positions in the small field camera 4the previously pinpointed locations. The operator observing the "smallfield" video monitor 14 does or does not confirm the defects and startsthe measuring procedure by means of the control member 16 of the opticalprobe 11. The values of the measurements and the location Of the defectsare recorded by the recorder 17 for drafting a control report.

In an automatic control installation corresponding to the diagram ofFIG. 5, the aforementioned equipment is supplemented by a dataprocessing system 19. This system controls the installation, processesthe video signals transmitted by the cameras 3 and 4, performs theautomatic diagnosis and analyses the measurements of the probe 11. Acontrol report is also drafted.

For automatic detection, the performance method can be summarized asfollows. A first rapid run takes place in accordance with a successionof exposures, whose field corresponds to that of the large field camera.The defects and remaining dust appear white on a grey background. Afirst processing by mathematical morphology consists of extracting thebackground from the image. The latter is then removed from the originalimage, which now only reveals the defects. A second run positions thedefects beneath the small field camera. Each image is processed in orderto accurately pinpoint these defects and position the probe. The probethen performs a profile plotting of the defect and a maximum calculationis performed on the series of measurements.

In exemplified manner an automatic control installation was produced onthe basis of the following components. An INTEL 80386 computer was usedat 25 MHz and to it was added a card permitting the acquisition andprocessing of 512×512 point images coded on 8 bits (i.e. 256 greylevels). The motorized stage permitting the large field displacement isconstituted by two perpendicular tables having a travel of 120 mm, aresolution of 1 μm and a displacement speed of 2 mm/s. The illuminationof the plate takes place by two cold light generators coupled to twooptical fibres, which gives four homogeneous light sources.

As the test plate dimensions were 70×780 mm, it was necessary to processeight large fields per face. Prior to each large field analysis, theplate was cleaned with alcohol and then blown with compressed air inorder to eliminate the maximum of traces and dust.

The aim of the large field analysis is to pinpoint defects which mayexist on the plate. The camera acquires a 100×70 mm field. On saidimage, the defects appear in white on a grey background. Thebinarization of the image (white defects on a black background) takesplace by mathematical morphology methods. The image is firstly eroded (Ntimes) and then expanded (N times). This gives image no. 2 from whichthe defects have been eliminated. This image is removed from the initialimage and then the result is thresholded. The number N of erosions andexpansions, as well as the height of the threshold are parameters whichcan easily be modified by the user.

The binarized image is filtered so as to eliminate spots with a surfacebelow a given threshold. If the thus obtained binary image reveals nodefect (white spots), the following field is brought under the cameraand the large field analysis phase is restarted. In the opposite case,the centre of each spot is calculated and then positioned beneath theobjective or lens of the small field camera.

The small field analysis is used for very accurately determining thedisplacement of the optical microprobe. The small field camera isequipped with a lens examining a field with a surface of 12×9 mm (i.e. aresolution of approximately 20 μm). Small field acquisition is performedin the same way as large field acquisition.

The processing of the small fields is in two different forms as afunction of the image type.

If the image has a large defect (wide scratch or large diameter hole),its histogram has two distinct peaks, the first in the low grey levelscorresponding to defects and the second in the white corresponding tothe background of the image. Thus, it is possible to calculate on thebasis of said histogram a threshold permitting the binarization of theimage in a global manner. In the opposite case, a local processing isperformed.

If the histogram has not permitted the calculation of a binarizationthreshold (defects too small, lack of contrast, inhomogeneity ofillumination), an area-based processing algorithm is used. Thisalgorithm functions by area comparison. Twenty five integrated,horizontal profiles of twelve lines and which partly overlap arecalculated on the image. These profiles make it possible to define amaximum and minimum profile of the image.

A thresholding on the difference of the maximim and minimum profilespermits the calculation of the position of the defect on the X axis. Avertical, integrated profile of the window [(Xd,0); (Xf,512)] is thencalculated on the Y axis. The window [(Xd,Yd); (Xf,Yf)] surrounding thedefect is thus accurately determined.

All the parts outside the window of the image are placed at zero andthen the windows are binarized. This gives a binary image as in the caseof global processing.

A defect is likened to a hole if its length/width ratio is between 1-aand 1+a, the value of a being a parameter of the system defined by theuser. In this case, two perpendicular depth measurements passing throughthe centre of the hole are calculated as a result of the displacementvia a motorized stage of an optical microprobe.

The defects which cannot be likened to holes are taken into account asscratches. The investigation of the depth of a scratch takes place bycalculating the maximum depth of a series of measurements. If the lengthof the scratch is below 5 mm, a profile is calculated every millimetre,otherwise every 2.5 mm.

Once the binarization of the small field image has taken place, ananalysis of the surface and roundness of the defects makes it possibleto classify them and select an optimum search mode for their depth.

We claim:
 1. Method for checking the surface state of one face (2) of asolid (1) for locating shape defects which may be present therein, saidmethod comprising the steps of:observing the face of the solid usingphotography in order to locate an area that may be a defect; observingsaid area using photography and optical magnification means; andmeasuring the size of the area with an optoelectronic probe; andcomparing the size of the area with a predetermined size limit todetermine whether the area is a defect, said area being considered adefect if the size of the area is larger than the predetermined sizelimit.
 2. Method according to claim 1, wherein the observation of theface of the solid takes place under a glancing, multidirectionalillumination.
 3. Method according to claim 1 wherein the steps ofobserving the face and the area by photography takes place in video andare done successively, and wherein the step of observing the facecomprises a first or large field analysis stage making it possible torapidly locate all areas that may be defects, and wherein the step ofobserving the area comprises a second or small field analysis stage onlyapplying to the areas detected in the first stage.
 4. Method accordingto claim 1 further comprising the step of recording the measurement madeby means of the optoelectronic probe (11).
 5. Device for checking thesurface state of one face (2) of a solid (1) for locating shape defects,said device comprising:means for receiving the solid (1) to permit apresentation of the face of the solid to be observed; means forilluminating said face; a large field video camera (3) for theobservation of said face; a small field video camera (4) for theobservation of said face; means for processing output signals suppliedby the video cameras, said processing means supplying information onshape defects that might be present on said face; an optoelectronicprobe (11); and means for controlling the optoelectronic probe, saidcontrolling means receiving the information from said processing means.6. Device according to claim 5 further comprising means for eliminatingdust which may be present on said face.
 7. Device according to claim 5wherein the receiving means comprise a translation plate member (12) fortranslating the solid, thereby permitting the displacement of said facein accordance with two crossed axes.
 8. Device according to claim 7wherein the translation plate member (12) ensures the displacement ofsaid face in accordance with one of the two axes for the large fieldvideo camera and in accordance with both axes for the small field videocamera and for the optoelectronic probe.
 9. Device according to claim 5wherein the means for processing the output signals supplied by thelarge field and small field video cameras (3, 4) comprise first andsecond monitors, said first monitor displaying a view filmed by thelarge field video camera and said second monitor displaying a viewfilmed by the small field video camera.
 10. Device according to claim 9further comprising means (15) for displaying values measured by theoptoelectronic probe.
 11. Device according to claim 9 wherein thereceiving means comprises a plate member for the translation of thesolid, thereby permitting the displacement of said face in accordancewith two crossed axes, and wherein the device further comprises means(18) for controlling said plate member in accordance with the axes whenthe device is operating either in a manual mode or in an automatic mode.12. Device according to claim 9 further comprising means (17) forrecording positions of areas that may be shape defects and for recordingmeasurements of the optoelectronic probe.
 13. Device according to claim5 further comprising a data processing control system (19) thatprocesses the output signals supplied by the large field and small fieldvideo cameras (3, 4), locates on the basis of said output signals areasthat may be shape defects, controls the optoelectronic probe (11) andanalyses measurements given by the optoelectronic probe.
 14. Method forchecking the surface state of one face (2) of a solid (1) for locatingshape defects which may be present therein, said method comprising thesteps of:observing the face of the solid using a large field videocamera in order to rapidly locate all areas that may be defects;observing the areas using a small field video camera and opticalmagnification means, said step of observing the areas occurringsubsequent to the step of observing the face and being limited only tothe areas located in the step of observing the face; and measuring thesize of the areas with an optoelectronic probe; and comparing the sizeof the areas with a predetermined size limit in order to determinewhether the areas are defects, any one of said areas being considered adefect if it is larger than the predetermined size limit.